/*
 * @(#)ObjectOutputStream.java	1.160 10/03/23
 *
 * Copyright (c) 2006, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.io;

import java.io.ObjectStreamClass.WeakClassKey;
import java.lang.ref.ReferenceQueue;
import java.security.AccessController;
import java.security.PrivilegedAction;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import static java.io.ObjectStreamClass.processQueue;

/**
 * An ObjectOutputStream writes primitive data types and graphs of Java objects
 * to an OutputStream.  The objects can be read (reconstituted) using an
 * ObjectInputStream.  Persistent storage of objects can be accomplished by
 * using a file for the stream.  If the stream is a network socket stream, the
 * objects can be reconstituted on another host or in another process.
 *
 * <p>Only objects that support the java.io.Serializable interface can be
 * written to streams.  The class of each serializable object is encoded
 * including the class name and signature of the class, the values of the
 * object's fields and arrays, and the closure of any other objects referenced
 * from the initial objects.
 *
 * <p>The method writeObject is used to write an object to the stream.  Any
 * object, including Strings and arrays, is written with writeObject. Multiple
 * objects or primitives can be written to the stream.  The objects must be
 * read back from the corresponding ObjectInputstream with the same types and
 * in the same order as they were written.
 *
 * <p>Primitive data types can also be written to the stream using the
 * appropriate methods from DataOutput. Strings can also be written using the
 * writeUTF method.
 *
 * <p>The default serialization mechanism for an object writes the class of the
 * object, the class signature, and the values of all non-transient and
 * non-static fields.  References to other objects (except in transient or
 * static fields) cause those objects to be written also. Multiple references
 * to a single object are encoded using a reference sharing mechanism so that
 * graphs of objects can be restored to the same shape as when the original was
 * written.
 *
 * <p>For example to write an object that can be read by the example in
 * ObjectInputStream:
 * <br>
 * <pre>
 *	FileOutputStream fos = new FileOutputStream("t.tmp");
 *	ObjectOutputStream oos = new ObjectOutputStream(fos);
 *
 *	oos.writeInt(12345);
 *	oos.writeObject("Today");
 *	oos.writeObject(new Date());
 *
 *	oos.close();
 * </pre>
 *
 * <p>Classes that require special handling during the serialization and
 * deserialization process must implement special methods with these exact
 * signatures:
 * <br>
 * <pre>
 * private void readObject(java.io.ObjectInputStream stream)
 *     throws IOException, ClassNotFoundException;
 * private void writeObject(java.io.ObjectOutputStream stream)
 *     throws IOException
 * private void readObjectNoData() 
 *     throws ObjectStreamException;
 * </pre>
 * 
 * <p>The writeObject method is responsible for writing the state of the object
 * for its particular class so that the corresponding readObject method can
 * restore it.  The method does not need to concern itself with the state
 * belonging to the object's superclasses or subclasses.  State is saved by
 * writing the individual fields to the ObjectOutputStream using the
 * writeObject method or by using the methods for primitive data types
 * supported by DataOutput.
 *
 * <p>Serialization does not write out the fields of any object that does not
 * implement the java.io.Serializable interface.  Subclasses of Objects that
 * are not serializable can be serializable. In this case the non-serializable
 * class must have a no-arg constructor to allow its fields to be initialized.
 * In this case it is the responsibility of the subclass to save and restore
 * the state of the non-serializable class. It is frequently the case that the
 * fields of that class are accessible (public, package, or protected) or that
 * there are get and set methods that can be used to restore the state.
 *
 * <p>Serialization of an object can be prevented by implementing writeObject
 * and readObject methods that throw the NotSerializableException.  The
 * exception will be caught by the ObjectOutputStream and abort the
 * serialization process.
 *
 * <p>Implementing the Externalizable interface allows the object to assume
 * complete control over the contents and format of the object's serialized
 * form.  The methods of the Externalizable interface, writeExternal and
 * readExternal, are called to save and restore the objects state.  When
 * implemented by a class they can write and read their own state using all of
 * the methods of ObjectOutput and ObjectInput.  It is the responsibility of
 * the objects to handle any versioning that occurs.
 *
 * <p>Enum constants are serialized differently than ordinary serializable or
 * externalizable objects.  The serialized form of an enum constant consists
 * solely of its name; field values of the constant are not transmitted.  To
 * serialize an enum constant, ObjectOutputStream writes the string returned by
 * the constant's name method.  Like other serializable or externalizable
 * objects, enum constants can function as the targets of back references
 * appearing subsequently in the serialization stream.  The process by which
 * enum constants are serialized cannot be customized; any class-specific
 * writeObject and writeReplace methods defined by enum types are ignored
 * during serialization.  Similarly, any serialPersistentFields or
 * serialVersionUID field declarations are also ignored--all enum types have a
 * fixed serialVersionUID of 0L.
 *
 * <p>Primitive data, excluding serializable fields and externalizable data, is
 * written to the ObjectOutputStream in block-data records. A block data record
 * is composed of a header and data. The block data header consists of a marker
 * and the number of bytes to follow the header.  Consecutive primitive data
 * writes are merged into one block-data record.  The blocking factor used for
 * a block-data record will be 1024 bytes.  Each block-data record will be
 * filled up to 1024 bytes, or be written whenever there is a termination of
 * block-data mode.  Calls to the ObjectOutputStream methods writeObject,
 * defaultWriteObject and writeFields initially terminate any existing
 * block-data record.
 *
 * @author	Mike Warres
 * @author	Roger Riggs
 * @version     1.160, 10/03/23
 * @see java.io.DataOutput
 * @see java.io.ObjectInputStream
 * @see java.io.Serializable
 * @see java.io.Externalizable
 * @see <a href="../../../platform/serialization/spec/output.html">Object Serialization Specification, Section 2, Object Output Classes</a>
 * @since       JDK1.1
 */
public class ObjectOutputStream
    extends OutputStream implements ObjectOutput, ObjectStreamConstants
{

    private static class Caches {
	/** cache of subclass security audit results */
	static final ConcurrentMap<WeakClassKey,Boolean> subclassAudits = 
	    new ConcurrentHashMap<WeakClassKey,Boolean>();

	/** queue for WeakReferences to audited subclasses */
	static final ReferenceQueue<Class<?>> subclassAuditsQueue = 
	    new ReferenceQueue<Class<?>>();
    }

    /** filter stream for handling block data conversion */
    private final BlockDataOutputStream bout;
    /** obj -> wire handle map */
    private final HandleTable handles;
    /** obj -> replacement obj map */
    private final ReplaceTable subs;
    /** stream protocol version */
    private int protocol = PROTOCOL_VERSION_2;
    /** recursion depth */
    private int depth;

    /** buffer for writing primitive field values */
    private byte[] primVals;

    /** if true, invoke writeObjectOverride() instead of writeObject() */
    private final boolean enableOverride;
    /** if true, invoke replaceObject() */
    private boolean enableReplace;

    // values below valid only during upcalls to writeObject()/writeExternal()
    /** object currently being serialized */
    private Object curObj;
    /** descriptor for current class (null if in writeExternal()) */
    private ObjectStreamClass curDesc;
    /** current PutField object */
    private PutFieldImpl curPut;

    /** custom storage for debug trace info */
    private final DebugTraceInfoStack debugInfoStack;

    /**
     * value of "sun.io.serialization.extendedDebugInfo" property,
     * as true or false for extended information about exception's place
     */
    private static final boolean extendedDebugInfo =
	((Boolean) java.security.AccessController.doPrivileged(
            new sun.security.action.GetBooleanAction(
		"sun.io.serialization.extendedDebugInfo"))).booleanValue();

    /**
     * Creates an ObjectOutputStream that writes to the specified OutputStream.
     * This constructor writes the serialization stream header to the
     * underlying stream; callers may wish to flush the stream immediately to
     * ensure that constructors for receiving ObjectInputStreams will not block
     * when reading the header.
     *
     * <p>If a security manager is installed, this constructor will check for
     * the "enableSubclassImplementation" SerializablePermission when invoked
     * directly or indirectly by the constructor of a subclass which overrides
     * the ObjectOutputStream.putFields or ObjectOutputStream.writeUnshared
     * methods.
     *
     * @param	out output stream to write to
     * @throws	IOException if an I/O error occurs while writing stream header
     * @throws	SecurityException if untrusted subclass illegally overrides
     * 		security-sensitive methods
     * @throws	NullPointerException if <code>out</code> is <code>null</code>
     * @since	1.4
     * @see	ObjectOutputStream#ObjectOutputStream()
     * @see	ObjectOutputStream#putFields()
     * @see	ObjectInputStream#ObjectInputStream(InputStream)
     */
    public ObjectOutputStream(OutputStream out) throws IOException {
	verifySubclass();
	bout = new BlockDataOutputStream(out);
	handles = new HandleTable(10, (float) 3.00);
	subs = new ReplaceTable(10, (float) 3.00);
	enableOverride = false;
	writeStreamHeader();
	bout.setBlockDataMode(true);
        if (extendedDebugInfo) {
	    debugInfoStack = new DebugTraceInfoStack();
	} else {
	    debugInfoStack = null;
        }   
    }

    /**
     * Provide a way for subclasses that are completely reimplementing
     * ObjectOutputStream to not have to allocate private data just used by
     * this implementation of ObjectOutputStream.
     *
     * <p>If there is a security manager installed, this method first calls the
     * security manager's <code>checkPermission</code> method with a
     * <code>SerializablePermission("enableSubclassImplementation")</code>
     * permission to ensure it's ok to enable subclassing.
     *
     * @throws	SecurityException if a security manager exists and its
     * 		<code>checkPermission</code> method denies enabling
     * 		subclassing.
     * @see SecurityManager#checkPermission
     * @see java.io.SerializablePermission
     */
    protected ObjectOutputStream() throws IOException, SecurityException {
	SecurityManager sm = System.getSecurityManager();
	if (sm != null) {
	    sm.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
	}
	bout = null;
	handles = null;
	subs = null;
	enableOverride = true;
        debugInfoStack = null;
    }

    /**
     * Specify stream protocol version to use when writing the stream.
     *
     * <p>This routine provides a hook to enable the current version of
     * Serialization to write in a format that is backwards compatible to a
     * previous version of the stream format.
     *
     * <p>Every effort will be made to avoid introducing additional
     * backwards incompatibilities; however, sometimes there is no
     * other alternative.
     *
     * @param	version use ProtocolVersion from java.io.ObjectStreamConstants.
     * @throws	IllegalStateException if called after any objects
     * 		have been serialized.
     * @throws	IllegalArgumentException if invalid version is passed in.
     * @throws	IOException if I/O errors occur
     * @see java.io.ObjectStreamConstants#PROTOCOL_VERSION_1
     * @see java.io.ObjectStreamConstants#PROTOCOL_VERSION_2
     * @since	1.2
     */
    public void useProtocolVersion(int version) throws IOException {
	if (handles.size() != 0) {
	    // REMIND: implement better check for pristine stream?
	    throw new IllegalStateException("stream non-empty");
	}
	switch (version) {
	    case PROTOCOL_VERSION_1:
	    case PROTOCOL_VERSION_2:
		protocol = version;
		break;
		
	    default:
		throw new IllegalArgumentException(
		    "unknown version: " + version);
	}
    }

    /**
     * Write the specified object to the ObjectOutputStream.  The class of the
     * object, the signature of the class, and the values of the non-transient
     * and non-static fields of the class and all of its supertypes are
     * written.  Default serialization for a class can be overridden using the
     * writeObject and the readObject methods.  Objects referenced by this
     * object are written transitively so that a complete equivalent graph of
     * objects can be reconstructed by an ObjectInputStream.
     *
     * <p>Exceptions are thrown for problems with the OutputStream and for
     * classes that should not be serialized.  All exceptions are fatal to the
     * OutputStream, which is left in an indeterminate state, and it is up to
     * the caller to ignore or recover the stream state.
     *
     * @throws	InvalidClassException Something is wrong with a class used by
     *		serialization.
     * @throws	NotSerializableException Some object to be serialized does not
     *		implement the java.io.Serializable interface.
     * @throws	IOException Any exception thrown by the underlying
     * 		OutputStream.
     */
    public final void writeObject(Object obj) throws IOException {
	if (enableOverride) {
	    writeObjectOverride(obj);
	    return;
	}
	try {
	    writeObject0(obj, false);
	} catch (IOException ex) {
	    if (depth == 0) {
		writeFatalException(ex);
	    }
	    throw ex;
	}
    }

    /**
     * Method used by subclasses to override the default writeObject method.
     * This method is called by trusted subclasses of ObjectInputStream that
     * constructed ObjectInputStream using the protected no-arg constructor.
     * The subclass is expected to provide an override method with the modifier
     * "final".
     *
     * @param	obj object to be written to the underlying stream
     * @throws	IOException if there are I/O errors while writing to the
     * 		underlying stream
     * @see #ObjectOutputStream()
     * @see #writeObject(Object)
     * @since 1.2
     */
    protected void writeObjectOverride(Object obj) throws IOException {
    }

    /**
     * Writes an "unshared" object to the ObjectOutputStream.  This method is
     * identical to writeObject, except that it always writes the given object
     * as a new, unique object in the stream (as opposed to a back-reference
     * pointing to a previously serialized instance).  Specifically:
     * <ul>
     *   <li>An object written via writeUnshared is always serialized in the
     *       same manner as a newly appearing object (an object that has not
     *       been written to the stream yet), regardless of whether or not the
     *       object has been written previously.
     *
     *   <li>If writeObject is used to write an object that has been previously
     *       written with writeUnshared, the previous writeUnshared operation
     *       is treated as if it were a write of a separate object.  In other
     *       words, ObjectOutputStream will never generate back-references to
     *       object data written by calls to writeUnshared.
     * </ul>
     * While writing an object via writeUnshared does not in itself guarantee a
     * unique reference to the object when it is deserialized, it allows a
     * single object to be defined multiple times in a stream, so that multiple
     * calls to readUnshared by the receiver will not conflict.  Note that the
     * rules described above only apply to the base-level object written with
     * writeUnshared, and not to any transitively referenced sub-objects in the
     * object graph to be serialized.
     *
     * <p>ObjectOutputStream subclasses which override this method can only be
     * constructed in security contexts possessing the
     * "enableSubclassImplementation" SerializablePermission; any attempt to
     * instantiate such a subclass without this permission will cause a
     * SecurityException to be thrown.
     *
     * @param   obj object to write to stream
     * @throws  NotSerializableException if an object in the graph to be
     *          serialized does not implement the Serializable interface
     * @throws  InvalidClassException if a problem exists with the class of an
     *          object to be serialized
     * @throws  IOException if an I/O error occurs during serialization
     * @since 1.4
     */
    public void writeUnshared(Object obj) throws IOException {
	try {
	    writeObject0(obj, true);
	} catch (IOException ex) {
	    if (depth == 0) {
		writeFatalException(ex);
	    }
	    throw ex;
	}
    }

    /**
     * Write the non-static and non-transient fields of the current class to
     * this stream.  This may only be called from the writeObject method of the
     * class being serialized. It will throw the NotActiveException if it is
     * called otherwise.
     *
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		<code>OutputStream</code>
     */
    public void defaultWriteObject() throws IOException {
	if (curObj == null || curDesc == null) {
	    throw new NotActiveException("not in call to writeObject");
	}
	bout.setBlockDataMode(false);
	defaultWriteFields(curObj, curDesc);
	bout.setBlockDataMode(true);
    }
    
    /**
     * Retrieve the object used to buffer persistent fields to be written to
     * the stream.  The fields will be written to the stream when writeFields
     * method is called.
     * 
     * @return	an instance of the class Putfield that holds the serializable
     * 		fields
     * @throws	IOException if I/O errors occur
     * @since 1.2
     */
    public ObjectOutputStream.PutField putFields() throws IOException {
	if (curPut == null) {
	    if (curObj == null || curDesc == null) {
		throw new NotActiveException("not in call to writeObject");
	    }
	    curPut = new PutFieldImpl(curDesc);
	}
	return curPut;
    }

    /**
     * Write the buffered fields to the stream.
     *
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     * @throws	NotActiveException Called when a classes writeObject method was
     * 		not called to write the state of the object.
     * @since 1.2
     */
    public void writeFields() throws IOException {
	if (curPut == null) {
	    throw new NotActiveException("no current PutField object");
	}
	bout.setBlockDataMode(false);
	curPut.writeFields();
	bout.setBlockDataMode(true);
    }

    /**
     * Reset will disregard the state of any objects already written to the
     * stream.  The state is reset to be the same as a new ObjectOutputStream.
     * The current point in the stream is marked as reset so the corresponding
     * ObjectInputStream will be reset at the same point.  Objects previously
     * written to the stream will not be refered to as already being in the
     * stream.  They will be written to the stream again.
     * 
     * @throws	IOException if reset() is invoked while serializing an object.
     */
    public void reset() throws IOException {
	if (depth != 0) {
	    throw new IOException("stream active");
	}
	bout.setBlockDataMode(false);
	bout.writeByte(TC_RESET);
	clear();
	bout.setBlockDataMode(true);
    }

    /**
     * Subclasses may implement this method to allow class data to be stored in
     * the stream. By default this method does nothing.  The corresponding
     * method in ObjectInputStream is resolveClass.  This method is called
     * exactly once for each unique class in the stream.  The class name and
     * signature will have already been written to the stream.  This method may
     * make free use of the ObjectOutputStream to save any representation of
     * the class it deems suitable (for example, the bytes of the class file).
     * The resolveClass method in the corresponding subclass of
     * ObjectInputStream must read and use any data or objects written by
     * annotateClass.
     *
     * @param	cl the class to annotate custom data for
     * @throws	IOException Any exception thrown by the underlying
     * 		OutputStream.
     */
    protected void annotateClass(Class<?> cl) throws IOException {
    }

    /**
     * Subclasses may implement this method to store custom data in the stream
     * along with descriptors for dynamic proxy classes.
     *
     * <p>This method is called exactly once for each unique proxy class
     * descriptor in the stream.  The default implementation of this method in
     * <code>ObjectOutputStream</code> does nothing.
     *
     * <p>The corresponding method in <code>ObjectInputStream</code> is
     * <code>resolveProxyClass</code>.  For a given subclass of
     * <code>ObjectOutputStream</code> that overrides this method, the
     * <code>resolveProxyClass</code> method in the corresponding subclass of
     * <code>ObjectInputStream</code> must read any data or objects written by
     * <code>annotateProxyClass</code>.
     *
     * @param	cl the proxy class to annotate custom data for
     * @throws	IOException any exception thrown by the underlying
     *		<code>OutputStream</code>
     * @see ObjectInputStream#resolveProxyClass(String[])
     * @since	1.3
     */
    protected void annotateProxyClass(Class<?> cl) throws IOException {
    }

    /** 
     * This method will allow trusted subclasses of ObjectOutputStream to
     * substitute one object for another during serialization. Replacing
     * objects is disabled until enableReplaceObject is called. The
     * enableReplaceObject method checks that the stream requesting to do
     * replacement can be trusted.  The first occurrence of each object written
     * into the serialization stream is passed to replaceObject.  Subsequent
     * references to the object are replaced by the object returned by the
     * original call to replaceObject.  To ensure that the private state of
     * objects is not unintentionally exposed, only trusted streams may use
     * replaceObject.
     * 
     * <p>The ObjectOutputStream.writeObject method takes a parameter of type
     * Object (as opposed to type Serializable) to allow for cases where
     * non-serializable objects are replaced by serializable ones.
     * 
     * <p>When a subclass is replacing objects it must insure that either a
     * complementary substitution must be made during deserialization or that
     * the substituted object is compatible with every field where the
     * reference will be stored.  Objects whose type is not a subclass of the
     * type of the field or array element abort the serialization by raising an
     * exception and the object is not be stored.
     *
     * <p>This method is called only once when each object is first
     * encountered.  All subsequent references to the object will be redirected
     * to the new object. This method should return the object to be
     * substituted or the original object.
     *
     * <p>Null can be returned as the object to be substituted, but may cause
     * NullReferenceException in classes that contain references to the
     * original object since they may be expecting an object instead of
     * null.
     *
     * @param	obj the object to be replaced
     * @return	the alternate object that replaced the specified one
     * @throws	IOException Any exception thrown by the underlying
     * 		OutputStream.
     */
    protected Object replaceObject(Object obj) throws IOException {
	return obj;
    }

    /**
     * Enable the stream to do replacement of objects in the stream.  When
     * enabled, the replaceObject method is called for every object being
     * serialized.
     *
     * <p>If <code>enable</code> is true, and there is a security manager
     * installed, this method first calls the security manager's
     * <code>checkPermission</code> method with a
     * <code>SerializablePermission("enableSubstitution")</code> permission to
     * ensure it's ok to enable the stream to do replacement of objects in the
     * stream.
     *
     * @param	enable boolean parameter to enable replacement of objects
     * @return	the previous setting before this method was invoked
     * @throws	SecurityException if a security manager exists and its
     * 		<code>checkPermission</code> method denies enabling the stream
     * 		to do replacement of objects in the stream.
     * @see SecurityManager#checkPermission
     * @see java.io.SerializablePermission
     */
    protected boolean enableReplaceObject(boolean enable)
	throws SecurityException
    {
	if (enable == enableReplace) {
	    return enable;
	}
	if (enable) {
	    SecurityManager sm = System.getSecurityManager();
	    if (sm != null) {
		sm.checkPermission(SUBSTITUTION_PERMISSION);
	    }
	}
	enableReplace = enable;
	return !enableReplace;
    }

    /**
     * The writeStreamHeader method is provided so subclasses can append or
     * prepend their own header to the stream.  It writes the magic number and
     * version to the stream.
     *
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    protected void writeStreamHeader() throws IOException {
	bout.writeShort(STREAM_MAGIC);
	bout.writeShort(STREAM_VERSION);
    }

    /**
     * Write the specified class descriptor to the ObjectOutputStream.  Class
     * descriptors are used to identify the classes of objects written to the
     * stream.  Subclasses of ObjectOutputStream may override this method to
     * customize the way in which class descriptors are written to the
     * serialization stream.  The corresponding method in ObjectInputStream,
     * <code>readClassDescriptor</code>, should then be overridden to
     * reconstitute the class descriptor from its custom stream representation.
     * By default, this method writes class descriptors according to the format
     * defined in the Object Serialization specification.
     * 
     * <p>Note that this method will only be called if the ObjectOutputStream
     * is not using the old serialization stream format (set by calling
     * ObjectOutputStream's <code>useProtocolVersion</code> method).  If this
     * serialization stream is using the old format
     * (<code>PROTOCOL_VERSION_1</code>), the class descriptor will be written
     * internally in a manner that cannot be overridden or customized.
     *
     * @param	desc class descriptor to write to the stream
     * @throws	IOException If an I/O error has occurred.
     * @see java.io.ObjectInputStream#readClassDescriptor()
     * @see #useProtocolVersion(int)
     * @see java.io.ObjectStreamConstants#PROTOCOL_VERSION_1
     * @since 1.3
     */
    protected void writeClassDescriptor(ObjectStreamClass desc)
	throws IOException
    {
	desc.writeNonProxy(this);
    }

    /**
     * Writes a byte. This method will block until the byte is actually
     * written.
     *
     * @param	val the byte to be written to the stream
     * @throws	IOException If an I/O error has occurred.
     */
    public void write(int val) throws IOException {
	bout.write(val);
    }

    /**
     * Writes an array of bytes. This method will block until the bytes are
     * actually written.
     *
     * @param	buf the data to be written
     * @throws	IOException If an I/O error has occurred.
     */
    public void write(byte[] buf) throws IOException {
	bout.write(buf, 0, buf.length, false);
    }

    /**
     * Writes a sub array of bytes.
     *
     * @param	buf the data to be written
     * @param	off the start offset in the data
     * @param	len the number of bytes that are written
     * @throws	IOException If an I/O error has occurred.
     */
    public void write(byte[] buf, int off, int len) throws IOException {
	if (buf == null) {
	    throw new NullPointerException();
	}
	int endoff = off + len;
	if (off < 0 || len < 0 || endoff > buf.length || endoff < 0) {
	    throw new IndexOutOfBoundsException();
	}
	bout.write(buf, off, len, false);
    }

    /**
     * Flushes the stream. This will write any buffered output bytes and flush
     * through to the underlying stream.
     *
     * @throws	IOException If an I/O error has occurred.
     */
    public void flush() throws IOException {
	bout.flush();
    }

    /**
     * Drain any buffered data in ObjectOutputStream.  Similar to flush but
     * does not propagate the flush to the underlying stream.
     *
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    protected void drain() throws IOException {
	bout.drain();
    }

    /**
     * Closes the stream. This method must be called to release any resources
     * associated with the stream.
     *
     * @throws	IOException If an I/O error has occurred.
     */
    public void close() throws IOException {
	flush();
	clear();
	bout.close();
    }

    /**
     * Writes a boolean.
     *
     * @param	val the boolean to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeBoolean(boolean val) throws IOException {
	bout.writeBoolean(val);
    }

    /**
     * Writes an 8 bit byte.
     *
     * @param	val the byte value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeByte(int val) throws IOException  {
	bout.writeByte(val);
    }

    /**
     * Writes a 16 bit short.
     *
     * @param	val the short value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeShort(int val)  throws IOException {
	bout.writeShort(val);
    }

    /**
     * Writes a 16 bit char.
     *
     * @param	val the char value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeChar(int val)  throws IOException {
	bout.writeChar(val);
    }

    /**
     * Writes a 32 bit int.
     *
     * @param	val the integer value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeInt(int val)  throws IOException {
	bout.writeInt(val);
    }

    /**
     * Writes a 64 bit long.
     *
     * @param	val the long value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeLong(long val)  throws IOException {
	bout.writeLong(val);
    }

    /**
     * Writes a 32 bit float.
     *
     * @param	val the float value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeFloat(float val) throws IOException {
	bout.writeFloat(val);
    }

    /**
     * Writes a 64 bit double.
     *
     * @param	val the double value to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeDouble(double val) throws IOException {
	bout.writeDouble(val);
    }

    /**
     * Writes a String as a sequence of bytes.
     *
     * @param	str the String of bytes to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeBytes(String str) throws IOException {
	bout.writeBytes(str);
    }

    /**
     * Writes a String as a sequence of chars.
     *
     * @param	str the String of chars to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeChars(String str) throws IOException {
	bout.writeChars(str);
    }

    /**
     * Primitive data write of this String in 
     * <a href="DataInput.html#modified-utf-8">modified UTF-8</a>
     * format.  Note that there is a
     * significant difference between writing a String into the stream as
     * primitive data or as an Object. A String instance written by writeObject
     * is written into the stream as a String initially. Future writeObject()
     * calls write references to the string into the stream.
     *
     * @param	str the String to be written
     * @throws	IOException if I/O errors occur while writing to the underlying
     * 		stream
     */
    public void writeUTF(String str) throws IOException {
	bout.writeUTF(str);
    }
    
    /**
     * Provide programmatic access to the persistent fields to be written
     * to ObjectOutput.
     *
     * @since 1.2
     */
    public static abstract class PutField {

	/**
	 * Put the value of the named boolean field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>boolean</code>
	 */
	public abstract void put(String name, boolean val);

	/**
	 * Put the value of the named byte field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>byte</code>
	 */
	public abstract void put(String name, byte val);

	/**
	 * Put the value of the named char field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>char</code>
	 */
	public abstract void put(String name, char val);

	/**
	 * Put the value of the named short field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>short</code>
	 */
	public abstract void put(String name, short val);

	/**
	 * Put the value of the named int field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>int</code>
	 */
	public abstract void put(String name, int val);

	/**
	 * Put the value of the named long field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>long</code>
	 */
	public abstract void put(String name, long val);

	/**
	 * Put the value of the named float field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>float</code>
	 */
	public abstract void put(String name, float val);

	/**
	 * Put the value of the named double field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not
         * <code>double</code>
	 */
	public abstract void put(String name, double val);

	/**
	 * Put the value of the named Object field into the persistent field.
	 *
	 * @param  name the name of the serializable field
	 * @param  val the value to assign to the field
         *         (which may be <code>null</code>)
         * @throws IllegalArgumentException if <code>name</code> does not
         * match the name of a serializable field for the class whose fields
         * are being written, or if the type of the named field is not a
         * reference type
	 */
	public abstract void put(String name, Object val);

	/**
	 * Write the data and fields to the specified ObjectOutput stream, 
         * which must be the same stream that produced this  
         * <code>PutField</code> object.
	 * 
	 * @param  out the stream to write the data and fields to
	 * @throws IOException if I/O errors occur while writing to the
	 * 	   underlying stream
	 * @throws IllegalArgumentException if the specified stream is not 
	 *         the same stream that produced this <code>PutField</code> 
	 *         object
	 * @deprecated This method does not write the values contained by this
	 * 	   <code>PutField</code> object in a proper format, and may
	 * 	   result in corruption of the serialization stream.  The
	 * 	   correct way to write <code>PutField</code> data is by
	 * 	   calling the {@link java.io.ObjectOutputStream#writeFields()}
	 * 	   method.
	 */
        @Deprecated
	public abstract void write(ObjectOutput out) throws IOException;
    }


    /**
     * Returns protocol version in use.
     */
    int getProtocolVersion() {
	return protocol;
    }

    /**
     * Writes string without allowing it to be replaced in stream.  Used by
     * ObjectStreamClass to write class descriptor type strings.
     */
    void writeTypeString(String str) throws IOException {
	int handle;
	if (str == null) {
	    writeNull();
	} else if ((handle = handles.lookup(str)) != -1) {
	    writeHandle(handle);
	} else {
	    writeString(str, false);
	}
    }
    
    /**
     * Verifies that this (possibly subclass) instance can be constructed
     * without violating security constraints: the subclass must not override
     * security-sensitive non-final methods, or else the
     * "enableSubclassImplementation" SerializablePermission is checked.
     */
    private void verifySubclass() {
	Class cl = getClass();
	if (cl == ObjectOutputStream.class) {
	    return;	
	}
	SecurityManager sm = System.getSecurityManager();
	if (sm == null) {
	    return;
	}
	processQueue(Caches.subclassAuditsQueue, Caches.subclassAudits);
	WeakClassKey key = new WeakClassKey(cl, Caches.subclassAuditsQueue);
	Boolean result = Caches.subclassAudits.get(key);
	if (result == null) {
	    result = Boolean.valueOf(auditSubclass(cl));
	    Caches.subclassAudits.putIfAbsent(key, result);
	}
	if (result.booleanValue()) {
	    return;
	}
	sm.checkPermission(SUBCLASS_IMPLEMENTATION_PERMISSION);
    }
    
    /**
     * Performs reflective checks on given subclass to verify that it doesn't
     * override security-sensitive non-final methods.  Returns true if subclass
     * is "safe", false otherwise.
     */
    private static boolean auditSubclass(final Class subcl) {
	Boolean result = (Boolean) AccessController.doPrivileged(
	    new PrivilegedAction() {
		public Object run() {
		    for (Class cl = subcl;
			 cl != ObjectOutputStream.class;
			 cl = cl.getSuperclass())
		    {
			try {
			    cl.getDeclaredMethod(
				"writeUnshared", new Class[] { Object.class });
			    return Boolean.FALSE;
			} catch (NoSuchMethodException ex) {
			}
			try {
			    cl.getDeclaredMethod("putFields", (Class[]) null);
			    return Boolean.FALSE;
			} catch (NoSuchMethodException ex) {
			}
		    }
		    return Boolean.TRUE;
		}
	    }
	);
	return result.booleanValue();
    }

    /**
     * Clears internal data structures.
     */
    private void clear() {
	subs.clear();
	handles.clear();
    }

    /**
     * Underlying writeObject/writeUnshared implementation.
     */
    private void writeObject0(Object obj, boolean unshared) 
	throws IOException 
    {
	boolean oldMode = bout.setBlockDataMode(false);
	depth++;
	try {
	    // handle previously written and non-replaceable objects
	    int h;
	    if ((obj = subs.lookup(obj)) == null) {
		writeNull();
		return;
	    } else if (!unshared && (h = handles.lookup(obj)) != -1) {
		writeHandle(h);
		return;
	    } else if (obj instanceof Class) {
		writeClass((Class) obj, unshared);
		return;
	    } else if (obj instanceof ObjectStreamClass) {
		writeClassDesc((ObjectStreamClass) obj, unshared);
		return;
	    }
	    
	    // check for replacement object
	    Object orig = obj;
	    Class cl = obj.getClass();
	    ObjectStreamClass desc;
	    for (;;) {
		// REMIND: skip this check for strings/arrays?
		Class repCl;
		desc = ObjectStreamClass.lookup(cl, true);
		if (!desc.hasWriteReplaceMethod() ||
		    (obj = desc.invokeWriteReplace(obj)) == null ||
		    (repCl = obj.getClass()) == cl)
		{
		    break;
		}
		cl = repCl;
	    }
	    if (enableReplace) {
		Object rep = replaceObject(obj);
		if (rep != obj && rep != null) {
		    cl = rep.getClass();
		    desc = ObjectStreamClass.lookup(cl, true);
		}
		obj = rep;
	    }

	    // if object replaced, run through original checks a second time
	    if (obj != orig) {
		subs.assign(orig, obj);
		if (obj == null) {
		    writeNull();
		    return;
		} else if (!unshared && (h = handles.lookup(obj)) != -1) {
		    writeHandle(h);
		    return;
		} else if (obj instanceof Class) {
		    writeClass((Class) obj, unshared);
		    return;
		} else if (obj instanceof ObjectStreamClass) {
		    writeClassDesc((ObjectStreamClass) obj, unshared);
		    return;
		}
	    }

	    // remaining cases
	    if (obj instanceof String) {
		writeString((String) obj, unshared);
	    } else if (cl.isArray()) {
		writeArray(obj, desc, unshared);
	    } else if (obj instanceof Enum) {
		writeEnum((Enum) obj, desc, unshared);
	    } else if (obj instanceof Serializable) {
		writeOrdinaryObject(obj, desc, unshared);
	    } else {
		if (extendedDebugInfo) {
		    throw new NotSerializableException(
			cl.getName() + "\n" + debugInfoStack.toString());
		} else {
		    throw new NotSerializableException(cl.getName());
		}    
	    }
	} finally {
	    depth--;
	    bout.setBlockDataMode(oldMode);
	}
    }
    
    /**
     * Writes null code to stream.
     */
    private void writeNull() throws IOException {
	bout.writeByte(TC_NULL);
    }
    
    /**
     * Writes given object handle to stream.
     */
    private void writeHandle(int handle) throws IOException {
	bout.writeByte(TC_REFERENCE);
	bout.writeInt(baseWireHandle + handle);
    }
    
    /**
     * Writes representation of given class to stream.
     */
    private void writeClass(Class cl, boolean unshared) throws IOException {
	bout.writeByte(TC_CLASS);
	writeClassDesc(ObjectStreamClass.lookup(cl, true), false);
	handles.assign(unshared ? null : cl);
    }
    
    /**
     * Writes representation of given class descriptor to stream.
     */
    private void writeClassDesc(ObjectStreamClass desc, boolean unshared) 
	throws IOException 
    {
	int handle;
	if (desc == null) {
	    writeNull();
	} else if (!unshared && (handle = handles.lookup(desc)) != -1) {
	    writeHandle(handle);
	} else if (desc.isProxy()) {
	    writeProxyDesc(desc, unshared);
	} else {
	    writeNonProxyDesc(desc, unshared);
	}
    }
    
    /**
     * Writes class descriptor representing a dynamic proxy class to stream.
     */
    private void writeProxyDesc(ObjectStreamClass desc, boolean unshared) 
	throws IOException 
    {
	bout.writeByte(TC_PROXYCLASSDESC);
	handles.assign(unshared ? null : desc);

	Class cl = desc.forClass();
	Class[] ifaces = cl.getInterfaces();
	bout.writeInt(ifaces.length);
	for (int i = 0; i < ifaces.length; i++) {
	    bout.writeUTF(ifaces[i].getName());
	}
	
	bout.setBlockDataMode(true);
	annotateProxyClass(cl);
	bout.setBlockDataMode(false);
	bout.writeByte(TC_ENDBLOCKDATA);
	
	writeClassDesc(desc.getSuperDesc(), false);
    }
    
    /**
     * Writes class descriptor representing a standard (i.e., not a dynamic
     * proxy) class to stream.
     */
    private void writeNonProxyDesc(ObjectStreamClass desc, boolean unshared) 
	throws IOException 
    {
	bout.writeByte(TC_CLASSDESC);
	handles.assign(unshared ? null : desc);
	
	if (protocol == PROTOCOL_VERSION_1) {
	    // do not invoke class descriptor write hook with old protocol
	    desc.writeNonProxy(this);
	} else {
	    writeClassDescriptor(desc);
	}
	
	Class cl = desc.forClass();
	bout.setBlockDataMode(true);
	annotateClass(cl);
	bout.setBlockDataMode(false);
	bout.writeByte(TC_ENDBLOCKDATA);
	
	writeClassDesc(desc.getSuperDesc(), false);
    }
    
    /**
     * Writes given string to stream, using standard or long UTF format
     * depending on string length.
     */
    private void writeString(String str, boolean unshared) throws IOException {
	handles.assign(unshared ? null : str);
	long utflen = bout.getUTFLength(str);
	if (utflen <= 0xFFFF) {
	    bout.writeByte(TC_STRING);
	    bout.writeUTF(str, utflen);
	} else {
	    bout.writeByte(TC_LONGSTRING);
	    bout.writeLongUTF(str, utflen);
	}
    }
    
    /**
     * Writes given array object to stream.
     */
    private void writeArray(Object array, 
			    ObjectStreamClass desc, 
			    boolean unshared) 
	throws IOException 
    {
	bout.writeByte(TC_ARRAY);
	writeClassDesc(desc, false);
	handles.assign(unshared ? null : array);
	
	Class ccl = desc.forClass().getComponentType();
	if (ccl.isPrimitive()) {
	    if (ccl == Integer.TYPE) {
		int[] ia = (int[]) array;
		bout.writeInt(ia.length);
		bout.writeInts(ia, 0, ia.length);
	    } else if (ccl == Byte.TYPE) {
		byte[] ba = (byte[]) array;
		bout.writeInt(ba.length);
		bout.write(ba, 0, ba.length, true);
	    } else if (ccl == Long.TYPE) {
		long[] ja = (long[]) array;
		bout.writeInt(ja.length);
		bout.writeLongs(ja, 0, ja.length);
	    } else if (ccl == Float.TYPE) {
		float[] fa = (float[]) array;
		bout.writeInt(fa.length);
		bout.writeFloats(fa, 0, fa.length);
	    } else if (ccl == Double.TYPE) {
		double[] da = (double[]) array;
		bout.writeInt(da.length);
		bout.writeDoubles(da, 0, da.length);
	    } else if (ccl == Short.TYPE) {
		short[] sa = (short[]) array;
		bout.writeInt(sa.length);
		bout.writeShorts(sa, 0, sa.length);
	    } else if (ccl == Character.TYPE) {
		char[] ca = (char[]) array;
		bout.writeInt(ca.length);
		bout.writeChars(ca, 0, ca.length);
	    } else if (ccl == Boolean.TYPE) {
		boolean[] za = (boolean[]) array;
		bout.writeInt(za.length);
		bout.writeBooleans(za, 0, za.length);
	    } else {
		throw new InternalError();
	    }
	} else {
	    Object[] objs = (Object[]) array;
	    int len = objs.length;
	    bout.writeInt(len);
	    if (extendedDebugInfo) {
		debugInfoStack.push(
		    "array (class \"" + array.getClass().getName() + 
                    "\", size: " + len  + ")");
	    }	
	    try {
		for (int i = 0; i < len; i++) {
		    if (extendedDebugInfo) {
			debugInfoStack.push(
			    "element of array (index: " + i + ")");
		    }	
		    try {
			writeObject0(objs[i], false);
		    } finally {
			if (extendedDebugInfo) {
			    debugInfoStack.pop();
			}     
		    }	
	        }
	    } finally {
      		if (extendedDebugInfo) {
       		    debugInfoStack.pop();
       		}  
	    }    
	}
    }
    
    /**
     * Writes given enum constant to stream.
     */
    private void writeEnum(Enum en,
			   ObjectStreamClass desc,
			   boolean unshared)
	throws IOException
    {
	bout.writeByte(TC_ENUM);
	ObjectStreamClass sdesc = desc.getSuperDesc();
	writeClassDesc((sdesc.forClass() == Enum.class) ? desc : sdesc, false);
	handles.assign(unshared ? null : en);
	writeString(en.name(), false);
    }

    /**
     * Writes representation of a "ordinary" (i.e., not a String, Class,
     * ObjectStreamClass, array, or enum constant) serializable object to the
     * stream.
     */
    private void writeOrdinaryObject(Object obj, 
				     ObjectStreamClass desc, 
				     boolean unshared) 
	throws IOException 
    {
        if (extendedDebugInfo) {
	    debugInfoStack.push(
		(depth == 1 ? "root " : "") + "object (class \"" + 
		obj.getClass().getName() + "\", " + obj.toString() + ")");
        }
        try {
	    desc.checkSerialize();

	    bout.writeByte(TC_OBJECT);
	    writeClassDesc(desc, false);
	    handles.assign(unshared ? null : obj);
	    if (desc.isExternalizable() && !desc.isProxy()) {
		writeExternalData((Externalizable) obj);
	    } else {
		writeSerialData(obj, desc);
	    }
	} finally {
    	    if (extendedDebugInfo) {
		debugInfoStack.pop();
	    }  
        }
    }
    
    /**
     * Writes externalizable data of given object by invoking its
     * writeExternal() method.
     */
    private void writeExternalData(Externalizable obj) throws IOException {
	Object oldObj = curObj;
	ObjectStreamClass oldDesc = curDesc;
	PutFieldImpl oldPut = curPut;
	curObj = obj;
	curDesc = null;
	curPut = null;
	
	if (extendedDebugInfo) {
	    debugInfoStack.push("writeExternal data");
	}	
	try {
	    if (protocol == PROTOCOL_VERSION_1) {
		obj.writeExternal(this);
	    } else {
		bout.setBlockDataMode(true);
		obj.writeExternal(this);
		bout.setBlockDataMode(false);
		bout.writeByte(TC_ENDBLOCKDATA);
	    }
	} finally {
	    if (extendedDebugInfo) {
		debugInfoStack.pop();
	    }     
	}	

	curObj = oldObj;
	curDesc = oldDesc;
	curPut = oldPut;
    }
    
    /**
     * Writes instance data for each serializable class of given object, from
     * superclass to subclass.
     */
    private void writeSerialData(Object obj, ObjectStreamClass desc) 
	throws IOException 
    {
	ObjectStreamClass.ClassDataSlot[] slots = desc.getClassDataLayout();
	for (int i = 0; i < slots.length; i++) {
	    ObjectStreamClass slotDesc = slots[i].desc;
	    if (slotDesc.hasWriteObjectMethod()) {
		Object oldObj = curObj;
		ObjectStreamClass oldDesc = curDesc;
		PutFieldImpl oldPut = curPut;
		curObj = obj;
		curDesc = slotDesc;
		curPut = null;

		if (extendedDebugInfo) {
		    debugInfoStack.push(
			"custom writeObject data (class \"" + 
			slotDesc.getName() + "\")");
		}	
		try {
		    bout.setBlockDataMode(true);
		    slotDesc.invokeWriteObject(obj, this);
		    bout.setBlockDataMode(false);
		    bout.writeByte(TC_ENDBLOCKDATA);
		} finally {
		    if (extendedDebugInfo) {
			debugInfoStack.pop();
		    }	
		} 

		curObj = oldObj;
		curDesc = oldDesc;
		curPut = oldPut;
	    } else {
		defaultWriteFields(obj, slotDesc);
	    }
	}
    }
    
    /**
     * Fetches and writes values of serializable fields of given object to
     * stream.  The given class descriptor specifies which field values to
     * write, and in which order they should be written.
     */
    private void defaultWriteFields(Object obj, ObjectStreamClass desc)
	throws IOException
    {
	// REMIND: perform conservative isInstance check here?
	desc.checkDefaultSerialize();

	int primDataSize = desc.getPrimDataSize();
	if (primVals == null || primVals.length < primDataSize) {
	    primVals = new byte[primDataSize];
	}
	desc.getPrimFieldValues(obj, primVals);
	bout.write(primVals, 0, primDataSize, false);
	
	ObjectStreamField[] fields = desc.getFields(false);
	Object[] objVals = new Object[desc.getNumObjFields()];
	int numPrimFields = fields.length - objVals.length;
	desc.getObjFieldValues(obj, objVals);
	for (int i = 0; i < objVals.length; i++) {
	    if (extendedDebugInfo) {
		debugInfoStack.push(
		    "field (class \"" + desc.getName() + "\", name: \"" + 
		    fields[numPrimFields + i].getName() + "\", type: \"" + 
		    fields[numPrimFields + i].getType() + "\")");
	    }	
	    try {
		writeObject0(objVals[i], 
			     fields[numPrimFields + i].isUnshared());
	    } finally {
		if (extendedDebugInfo) {
		    debugInfoStack.pop();
		}     
	    }	
	}
    }

    /**
     * Attempts to write to stream fatal IOException that has caused
     * serialization to abort.
     */
    private void writeFatalException(IOException ex) throws IOException {
	/*
	 * Note: the serialization specification states that if a second
	 * IOException occurs while attempting to serialize the original fatal
	 * exception to the stream, then a StreamCorruptedException should be
	 * thrown (section 2.1).  However, due to a bug in previous
	 * implementations of serialization, StreamCorruptedExceptions were
	 * rarely (if ever) actually thrown--the "root" exceptions from
	 * underlying streams were thrown instead.  This historical behavior is
	 * followed here for consistency.
	 */
	clear();
	boolean oldMode = bout.setBlockDataMode(false);
	try {
	    bout.writeByte(TC_EXCEPTION);
	    writeObject0(ex, false);
	    clear();
	} finally {
	    bout.setBlockDataMode(oldMode);
	}
    }

    /**
     * Converts specified span of float values into byte values.
     */
    // REMIND: remove once hotspot inlines Float.floatToIntBits
    private static native void floatsToBytes(float[] src, int srcpos, 
					     byte[] dst, int dstpos, 
					     int nfloats);

    /**
     * Converts specified span of double values into byte values.
     */
    // REMIND: remove once hotspot inlines Double.doubleToLongBits
    private static native void doublesToBytes(double[] src, int srcpos,
					      byte[] dst, int dstpos, 
					      int ndoubles);

    /**
     * Default PutField implementation.
     */
    private class PutFieldImpl extends PutField {

	/** class descriptor describing serializable fields */
	private final ObjectStreamClass desc;
	/** primitive field values */
	private final byte[] primVals;
	/** object field values */
	private final Object[] objVals;

	/**
	 * Creates PutFieldImpl object for writing fields defined in given
	 * class descriptor.
	 */
	PutFieldImpl(ObjectStreamClass desc) {
	    this.desc = desc;
	    primVals = new byte[desc.getPrimDataSize()];
	    objVals = new Object[desc.getNumObjFields()];
	}

	public void put(String name, boolean val) {
	    Bits.putBoolean(primVals, getFieldOffset(name, Boolean.TYPE), val);
	}

	public void put(String name, byte val) {
	    primVals[getFieldOffset(name, Byte.TYPE)] = val;
	}

	public void put(String name, char val) {
	    Bits.putChar(primVals, getFieldOffset(name, Character.TYPE), val);
	}

	public void put(String name, short val) {
	    Bits.putShort(primVals, getFieldOffset(name, Short.TYPE), val);
	}

	public void put(String name, int val) {
	    Bits.putInt(primVals, getFieldOffset(name, Integer.TYPE), val);
	}

	public void put(String name, float val) {
	    Bits.putFloat(primVals, getFieldOffset(name, Float.TYPE), val);
	}

	public void put(String name, long val) {
	    Bits.putLong(primVals, getFieldOffset(name, Long.TYPE), val);
	}

	public void put(String name, double val) {
	    Bits.putDouble(primVals, getFieldOffset(name, Double.TYPE), val);
	}

	public void put(String name, Object val) {
	    objVals[getFieldOffset(name, Object.class)] = val;
	}

	// deprecated in ObjectOutputStream.PutField
	public void write(ObjectOutput out) throws IOException {
	    /*
	     * Applications should *not* use this method to write PutField
	     * data, as it will lead to stream corruption if the PutField
	     * object writes any primitive data (since block data mode is not
	     * unset/set properly, as is done in OOS.writeFields()).  This
	     * broken implementation is being retained solely for behavioral
	     * compatibility, in order to support applications which use
	     * OOS.PutField.write() for writing only non-primitive data.
	     * 
	     * Serialization of unshared objects is not implemented here since
	     * it is not necessary for backwards compatibility; also, unshared
	     * semantics may not be supported by the given ObjectOutput
	     * instance.  Applications which write unshared objects using the
	     * PutField API must use OOS.writeFields().
	     */
	    if (ObjectOutputStream.this != out) {
		throw new IllegalArgumentException("wrong stream");
	    }
	    out.write(primVals, 0, primVals.length);
	    
	    ObjectStreamField[] fields = desc.getFields(false);
	    int numPrimFields = fields.length - objVals.length;
	    // REMIND: warn if numPrimFields > 0?
	    for (int i = 0; i < objVals.length; i++) {
		if (fields[numPrimFields + i].isUnshared()) {
		    throw new IOException("cannot write unshared object");
		}
		out.writeObject(objVals[i]);
	    }
	}
	
	/**
	 * Writes buffered primitive data and object fields to stream.
	 */
	void writeFields() throws IOException {
	    bout.write(primVals, 0, primVals.length, false);

	    ObjectStreamField[] fields = desc.getFields(false);
	    int numPrimFields = fields.length - objVals.length;
	    for (int i = 0; i < objVals.length; i++) {
		if (extendedDebugInfo) {
		    debugInfoStack.push(
			"field (class \"" + desc.getName() + "\", name: \"" + 
			fields[numPrimFields + i].getName() + "\", type: \"" + 
			fields[numPrimFields + i].getType() + "\")");
		}	
		try {
		    writeObject0(objVals[i], 
				 fields[numPrimFields + i].isUnshared());
		} finally {
		    if (extendedDebugInfo) {
			debugInfoStack.pop();
		    }	
		}     
	    }
	}

	/**
	 * Returns offset of field with given name and type.  A specified type
	 * of null matches all types, Object.class matches all non-primitive
	 * types, and any other non-null type matches assignable types only.
	 * Throws IllegalArgumentException if no matching field found.
	 */
	private int getFieldOffset(String name, Class type) {
	    ObjectStreamField field = desc.getField(name, type);
	    if (field == null) {
                throw new IllegalArgumentException("no such field " + name +
						   " with type " + type);
	    }
	    return field.getOffset();
	}
    }

    /**
     * Buffered output stream with two modes: in default mode, outputs data in
     * same format as DataOutputStream; in "block data" mode, outputs data
     * bracketed by block data markers (see object serialization specification
     * for details).
     */
    private static class BlockDataOutputStream 
	extends OutputStream implements DataOutput
    {
	/** maximum data block length */
	private static final int MAX_BLOCK_SIZE = 1024;
	/** maximum data block header length */
	private static final int MAX_HEADER_SIZE = 5;
	/** (tunable) length of char buffer (for writing strings) */
	private static final int CHAR_BUF_SIZE = 256;

	/** buffer for writing general/block data */
	private final byte[] buf = new byte[MAX_BLOCK_SIZE];
	/** buffer for writing block data headers */
	private final byte[] hbuf = new byte[MAX_HEADER_SIZE];
	/** char buffer for fast string writes */
	private final char[] cbuf = new char[CHAR_BUF_SIZE];
	
	/** block data mode */
	private boolean blkmode = false;
	/** current offset into buf */
	private int pos = 0;

	/** underlying output stream */
	private final OutputStream out;
	/** loopback stream (for data writes that span data blocks) */
	private final DataOutputStream dout;

	/**
	 * Creates new BlockDataOutputStream on top of given underlying stream.
	 * Block data mode is turned off by default.
	 */
	BlockDataOutputStream(OutputStream out) {
	    this.out = out;
	    dout = new DataOutputStream(this);
	}

	/**
	 * Sets block data mode to the given mode (true == on, false == off)
	 * and returns the previous mode value.  If the new mode is the same as
	 * the old mode, no action is taken.  If the new mode differs from the
	 * old mode, any buffered data is flushed before switching to the new
	 * mode.
	 */
	boolean setBlockDataMode(boolean mode) throws IOException {
	    if (blkmode == mode) {
		return blkmode;
	    }
	    drain();
	    blkmode = mode;
	    return !blkmode;
	}

	/**
	 * Returns true if the stream is currently in block data mode, false
	 * otherwise.
	 */
	boolean getBlockDataMode() {
	    return blkmode;
	}

	/* ----------------- generic output stream methods ----------------- */
	/*
	 * The following methods are equivalent to their counterparts in
	 * OutputStream, except that they partition written data into data
	 * blocks when in block data mode.
	 */

	public void write(int b) throws IOException {
	    if (pos >= MAX_BLOCK_SIZE) {
		drain();
	    }
	    buf[pos++] = (byte) b;
	}

	public void write(byte[] b) throws IOException {
	    write(b, 0, b.length, false);
	}

	public void write(byte[] b, int off, int len) throws IOException {
	    write(b, off, len, false);
	}

	public void flush() throws IOException {
	    drain();
	    out.flush();
	}

	public void close() throws IOException {
	    flush();
	    out.close();
	}

	/**
	 * Writes specified span of byte values from given array.  If copy is
	 * true, copies the values to an intermediate buffer before writing
	 * them to underlying stream (to avoid exposing a reference to the
	 * original byte array).
	 */
	void write(byte[] b, int off, int len, boolean copy) 
	    throws IOException 
	{
	    if (!(copy || blkmode)) { 		// write directly
		drain();
		out.write(b, off, len);
		return;
	    }

	    while (len > 0) {
		if (pos >= MAX_BLOCK_SIZE) {
		    drain();
		}
		if (len >= MAX_BLOCK_SIZE && !copy && pos == 0) {
		    // avoid unnecessary copy
		    writeBlockHeader(MAX_BLOCK_SIZE);
		    out.write(b, off, MAX_BLOCK_SIZE);
		    off += MAX_BLOCK_SIZE;
		    len -= MAX_BLOCK_SIZE;
		} else {
		    int wlen = Math.min(len, MAX_BLOCK_SIZE - pos);
		    System.arraycopy(b, off, buf, pos, wlen);
		    pos += wlen;
		    off += wlen;
		    len -= wlen;
		}
	    }
	}

	/**
	 * Writes all buffered data from this stream to the underlying stream,
	 * but does not flush underlying stream.
	 */
	void drain() throws IOException {
	    if (pos == 0) {
		return;
	    }
	    if (blkmode) {
		writeBlockHeader(pos);
	    }
	    out.write(buf, 0, pos);
	    pos = 0;
	}

	/**
	 * Writes block data header.  Data blocks shorter than 256 bytes are
	 * prefixed with a 2-byte header; all others start with a 5-byte
	 * header.
	 */
	private void writeBlockHeader(int len) throws IOException {
	    if (len <= 0xFF) {
		hbuf[0] = TC_BLOCKDATA;
		hbuf[1] = (byte) len;
		out.write(hbuf, 0, 2);
	    } else {
		hbuf[0] = TC_BLOCKDATALONG;
		Bits.putInt(hbuf, 1, len);
		out.write(hbuf, 0, 5);
	    }
	}


	/* ----------------- primitive data output methods ----------------- */
	/*
	 * The following methods are equivalent to their counterparts in
	 * DataOutputStream, except that they partition written data into data
	 * blocks when in block data mode.
	 */

	public void writeBoolean(boolean v) throws IOException {
	    if (pos >= MAX_BLOCK_SIZE) {
		drain();
	    }
	    Bits.putBoolean(buf, pos++, v);
	}

	public void writeByte(int v) throws IOException {
	    if (pos >= MAX_BLOCK_SIZE) {
		drain();
	    }
	    buf[pos++] = (byte) v;
	}

	public void writeChar(int v) throws IOException {
	    if (pos + 2 <= MAX_BLOCK_SIZE) {
		Bits.putChar(buf, pos, (char) v);
		pos += 2;
	    } else {
		dout.writeChar(v);
	    }
	}

	public void writeShort(int v) throws IOException {
	    if (pos + 2 <= MAX_BLOCK_SIZE) {
		Bits.putShort(buf, pos, (short) v);
		pos += 2;
	    } else {
		dout.writeShort(v);
	    }
	}

	public void writeInt(int v) throws IOException {
	    if (pos + 4 <= MAX_BLOCK_SIZE) {
		Bits.putInt(buf, pos, v);
		pos += 4;
	    } else {
		dout.writeInt(v);
	    }
	}

	public void writeFloat(float v) throws IOException {
	    if (pos + 4 <= MAX_BLOCK_SIZE) {
		Bits.putFloat(buf, pos, v);
		pos += 4;
	    } else {
		dout.writeFloat(v);
	    }
	}

	public void writeLong(long v) throws IOException {
	    if (pos + 8 <= MAX_BLOCK_SIZE) {
		Bits.putLong(buf, pos, v);
		pos += 8;
	    } else {
		dout.writeLong(v);
	    }
	}

	public void writeDouble(double v) throws IOException {
	    if (pos + 8 <= MAX_BLOCK_SIZE) {
		Bits.putDouble(buf, pos, v);
		pos += 8;
	    } else {
		dout.writeDouble(v);
	    }
	}

	public void writeBytes(String s) throws IOException {
	    int endoff = s.length();
	    int cpos = 0;
	    int csize = 0;
	    for (int off = 0; off < endoff; ) {
		if (cpos >= csize) {
		    cpos = 0;
		    csize = Math.min(endoff - off, CHAR_BUF_SIZE);
		    s.getChars(off, off + csize, cbuf, 0);
		} 
		if (pos >= MAX_BLOCK_SIZE) {
		    drain();
		}
		int n = Math.min(csize - cpos, MAX_BLOCK_SIZE - pos);
		int stop = pos + n;
		while (pos < stop) {
		    buf[pos++] = (byte) cbuf[cpos++];
		}
		off += n;
	    }
	}

	public void writeChars(String s) throws IOException {
	    int endoff = s.length();
	    for (int off = 0; off < endoff; ) {
		int csize = Math.min(endoff - off, CHAR_BUF_SIZE);
		s.getChars(off, off + csize, cbuf, 0);
		writeChars(cbuf, 0, csize);
		off += csize;
	    }
	}
	
	public void writeUTF(String s) throws IOException {
	    writeUTF(s, getUTFLength(s));
	}


	/* -------------- primitive data array output methods -------------- */
	/*
	 * The following methods write out spans of primitive data values.
	 * Though equivalent to calling the corresponding primitive write
	 * methods repeatedly, these methods are optimized for writing groups
	 * of primitive data values more efficiently.
	 */

	void writeBooleans(boolean[] v, int off, int len) throws IOException {
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos >= MAX_BLOCK_SIZE) {
		    drain();
		}
		int stop = Math.min(endoff, off + (MAX_BLOCK_SIZE - pos));
		while (off < stop) {
		    Bits.putBoolean(buf, pos++, v[off++]);
		}
	    }
	}

	void writeChars(char[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 2;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 1;
		    int stop = Math.min(endoff, off + avail);
		    while (off < stop) {
			Bits.putChar(buf, pos, v[off++]);
			pos += 2;
		    }
		} else {
		    dout.writeChar(v[off++]);
		}
	    }
	}

	void writeShorts(short[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 2;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 1;
		    int stop = Math.min(endoff, off + avail);
		    while (off < stop) {
			Bits.putShort(buf, pos, v[off++]);
			pos += 2;
		    }
		} else {
		    dout.writeShort(v[off++]);
		}
	    }
	}

	void writeInts(int[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 4;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 2;
		    int stop = Math.min(endoff, off + avail);
		    while (off < stop) {
			Bits.putInt(buf, pos, v[off++]);
			pos += 4;
		    }
		} else {
		    dout.writeInt(v[off++]);
		}
	    }
	}

	void writeFloats(float[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 4;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 2;
		    int chunklen = Math.min(endoff - off, avail);
		    floatsToBytes(v, off, buf, pos, chunklen);
		    off += chunklen;
		    pos += chunklen << 2;
		} else {
		    dout.writeFloat(v[off++]);
		}
	    }
	}

	void writeLongs(long[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 8;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 3;
		    int stop = Math.min(endoff, off + avail);
		    while (off < stop) {
			Bits.putLong(buf, pos, v[off++]);
			pos += 8;
		    }
		} else {
		    dout.writeLong(v[off++]);
		}
	    }
	}

	void writeDoubles(double[] v, int off, int len) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 8;
	    int endoff = off + len;
	    while (off < endoff) {
		if (pos <= limit) {
		    int avail = (MAX_BLOCK_SIZE - pos) >> 3;
		    int chunklen = Math.min(endoff - off, avail);
		    doublesToBytes(v, off, buf, pos, chunklen);
		    off += chunklen;
		    pos += chunklen << 3;
		} else {
		    dout.writeDouble(v[off++]);
		}
	    }
	}

	/**
	 * Returns the length in bytes of the UTF encoding of the given string.
	 */
	long getUTFLength(String s) {
	    int len = s.length();
	    long utflen = 0;
	    for (int off = 0; off < len; ) {
		int csize = Math.min(len - off, CHAR_BUF_SIZE);
		s.getChars(off, off + csize, cbuf, 0);
		for (int cpos = 0; cpos < csize; cpos++) {
		    char c = cbuf[cpos];
		    if (c >= 0x0001 && c <= 0x007F) {
			utflen++;
		    } else if (c > 0x07FF) {
			utflen += 3;
		    } else {
			utflen += 2;
		    }
		}
		off += csize;
	    }
	    return utflen;
	}

	/**
	 * Writes the given string in UTF format.  This method is used in
	 * situations where the UTF encoding length of the string is already
	 * known; specifying it explicitly avoids a prescan of the string to
	 * determine its UTF length.
	 */
	void writeUTF(String s, long utflen) throws IOException {
	    if (utflen > 0xFFFFL) {
		throw new UTFDataFormatException();
	    }
	    writeShort((int) utflen);
	    if (utflen == (long) s.length()) {
		writeBytes(s);
	    } else {
		writeUTFBody(s);
	    }
	}

	/**
	 * Writes given string in "long" UTF format.  "Long" UTF format is
	 * identical to standard UTF, except that it uses an 8 byte header
	 * (instead of the standard 2 bytes) to convey the UTF encoding length.
	 */
	void writeLongUTF(String s) throws IOException {
	    writeLongUTF(s, getUTFLength(s));
	}

	/**
	 * Writes given string in "long" UTF format, where the UTF encoding
	 * length of the string is already known.
	 */
	void writeLongUTF(String s, long utflen) throws IOException {
	    writeLong(utflen);
	    if (utflen == (long) s.length()) {
		writeBytes(s);
	    } else {
		writeUTFBody(s);
	    }
	}

	/**
	 * Writes the "body" (i.e., the UTF representation minus the 2-byte or
	 * 8-byte length header) of the UTF encoding for the given string.
	 */
	private void writeUTFBody(String s) throws IOException {
	    int limit = MAX_BLOCK_SIZE - 3;
	    int len = s.length();
	    for (int off = 0; off < len; ) {
		int csize = Math.min(len - off, CHAR_BUF_SIZE);
		s.getChars(off, off + csize, cbuf, 0);
		for (int cpos = 0; cpos < csize; cpos++) {
		    char c = cbuf[cpos];
		    if (pos <= limit) {
			if (c <= 0x007F && c != 0) {
			    buf[pos++] = (byte) c;
			} else if (c > 0x07FF) {
			    buf[pos + 2] = (byte) (0x80 | ((c >> 0) & 0x3F));
			    buf[pos + 1] = (byte) (0x80 | ((c >> 6) & 0x3F));
			    buf[pos + 0] = (byte) (0xE0 | ((c >> 12) & 0x0F));
			    pos += 3;
			} else {
			    buf[pos + 1] = (byte) (0x80 | ((c >> 0) & 0x3F));
			    buf[pos + 0] = (byte) (0xC0 | ((c >> 6) & 0x1F));
			    pos += 2;
			}
		    } else { 	// write one byte at a time to normalize block
			if (c <= 0x007F && c != 0) {
			    write(c);
			} else if (c > 0x07FF) {
			    write(0xE0 | ((c >> 12) & 0x0F));
			    write(0x80 | ((c >> 6) & 0x3F));
			    write(0x80 | ((c >> 0) & 0x3F));
			} else {
			    write(0xC0 | ((c >> 6) & 0x1F));
			    write(0x80 | ((c >> 0) & 0x3F));
			}
		    }
		}
		off += csize;
	    }
	}
    }

    /**
     * Lightweight identity hash table which maps objects to integer handles,
     * assigned in ascending order.
     */
    private static class HandleTable {

	/* number of mappings in table/next available handle */
	private int size;
	/* size threshold determining when to expand hash spine */
	private int threshold;
	/* factor for computing size threshold */
	private final float loadFactor;
	/* maps hash value -> candidate handle value */
	private int[] spine;
	/* maps handle value -> next candidate handle value */
	private int[] next;
	/* maps handle value -> associated object */
	private Object[] objs;

	/**
	 * Creates new HandleTable with given capacity and load factor.
	 */
	HandleTable(int initialCapacity, float loadFactor) {
	    this.loadFactor = loadFactor;
	    spine = new int[initialCapacity];
	    next = new int[initialCapacity];
	    objs = new Object[initialCapacity];
	    threshold = (int) (initialCapacity * loadFactor);
	    clear();
	}

	/**
	 * Assigns next available handle to given object, and returns handle
	 * value.  Handles are assigned in ascending order starting at 0.
	 */
	int assign(Object obj) {
	    if (size >= next.length) {
		growEntries();
	    }
	    if (size >= threshold) {
		growSpine();
	    }
	    insert(obj, size);
	    return size++;
	}

	/**
	 * Looks up and returns handle associated with given object, or -1 if
	 * no mapping found.
	 */
	int lookup(Object obj) {
	    if (size == 0) {
		return -1;
	    }
	    int index = hash(obj) % spine.length;
	    for (int i = spine[index]; i >= 0; i = next[i]) {
		if (objs[i] == obj) {
		    return i;
		}
	    }
	    return -1;
	}

	/**
	 * Resets table to its initial (empty) state.
	 */
	void clear() {
	    Arrays.fill(spine, -1);
	    Arrays.fill(objs, 0, size, null);
	    size = 0;
	}

	/**
	 * Returns the number of mappings currently in table.
	 */
	int size() {
	    return size;
	}

	/**
	 * Inserts mapping object -> handle mapping into table.  Assumes table
	 * is large enough to accommodate new mapping.
	 */
	private void insert(Object obj, int handle) {
	    int index = hash(obj) % spine.length;
	    objs[handle] = obj;
	    next[handle] = spine[index];
	    spine[index] = handle;
	}

	/**
	 * Expands the hash "spine" -- equivalent to increasing the number of
	 * buckets in a conventional hash table.
	 */
	private void growSpine() {
	    spine = new int[(spine.length << 1) + 1];
	    threshold = (int) (spine.length * loadFactor);
	    Arrays.fill(spine, -1);
	    for (int i = 0; i < size; i++) {
		insert(objs[i], i);
	    }
	}

	/**
	 * Increases hash table capacity by lengthening entry arrays.
	 */
	private void growEntries() {
	    int newLength = (next.length << 1) + 1;
	    int[] newNext = new int[newLength];
	    System.arraycopy(next, 0, newNext, 0, size);
	    next = newNext;

	    Object[] newObjs = new Object[newLength];
	    System.arraycopy(objs, 0, newObjs, 0, size);
	    objs = newObjs;
	}

	/**
	 * Returns hash value for given object.
	 */
	private int hash(Object obj) {
	    return System.identityHashCode(obj) & 0x7FFFFFFF;
	}
    }

    /**
     * Lightweight identity hash table which maps objects to replacement
     * objects.
     */
    private static class ReplaceTable {

	/* maps object -> index */
	private final HandleTable htab;
	/* maps index -> replacement object */
	private Object[] reps;

	/**
	 * Creates new ReplaceTable with given capacity and load factor.
	 */
	ReplaceTable(int initialCapacity, float loadFactor) {
	    htab = new HandleTable(initialCapacity, loadFactor);
	    reps = new Object[initialCapacity];
	}

	/**
	 * Enters mapping from object to replacement object.
	 */
	void assign(Object obj, Object rep) {
	    int index = htab.assign(obj);
	    while (index >= reps.length) {
		grow();
	    }
	    reps[index] = rep;
	}

	/**
	 * Looks up and returns replacement for given object.  If no
	 * replacement is found, returns the lookup object itself.
	 */
	Object lookup(Object obj) {
	    int index = htab.lookup(obj);
	    return (index >= 0) ? reps[index] : obj;
	}

	/**
	 * Resets table to its initial (empty) state.
	 */
	void clear() {
	    Arrays.fill(reps, 0, htab.size(), null);
	    htab.clear();
	}

	/**
	 * Returns the number of mappings currently in table.
	 */
	int size() {
	    return htab.size();
	}
	
	/**
	 * Increases table capacity.
	 */
	private void grow() {
	    Object[] newReps = new Object[(reps.length << 1) + 1];
	    System.arraycopy(reps, 0, newReps, 0, reps.length);
	    reps = newReps;
	}
    }

    /**
     * Stack to keep debug information about the state of the
     * serialization process, for embedding in exception messages.
     */   
    private static class DebugTraceInfoStack {
        private final List<String> stack;

        DebugTraceInfoStack() {
            stack = new ArrayList<String>();
        }

	/**
	 * Removes all of the elements from enclosed list. 
	 */
        void clear() {
            stack.clear();
        }

	/**
	 * Removes the object at the top of enclosed list.
	 */
        void pop() {
	    stack.remove(stack.size()-1);
        }

	/**
	 * Pushes a String onto the top of enclosed list.
	 */
        void push(String entry) {
            stack.add("\t- " + entry);          
        }

	/**
	 * Returns a string representation of this object
	 */
        public String toString() {
	    StringBuilder buffer = new StringBuilder();
	    if (!stack.isEmpty()) {
		for(int i = stack.size(); i > 0; i-- ) {
		    buffer.append(stack.get(i-1) + ((i != 1) ? "\n" : ""));
		}
	    }
            return buffer.toString();
        }
    }   

}
